# Physical constants in nature

1. Nov 4, 2004

### welcher_weg

hallo guys. this is my first post( welcome me :tongue2: !). Anyway i'm a freshman taking electrical engineering with a keen intrest in physics.

anyway my question is this: How do we know that physical constants are really constants.

Ok, one way you might say is by assuming, that the quantities are linearly dependant on each other. And then verifying this via experimentation. But how do we know that these are not just linearly correleated at human scales. may be at a quantum level(or stellar sizes) the quantities may vary?(i specifically have G in mind when i talk abt this)

any reply will be greatly appreaciated. thankyou.

2. Nov 4, 2004

Welcome!

How do we know that constants are really constant? I don't think we do! The constants we have such as speed of light and big 'G' appear to be constant everywhere in the Universe, but this doesn't mean that they ARE constant. Could the speed of light be increasing/decreasing by a tiny amount? Could be. It is interesting that most of the attempts to measure c accurately over the past hundred years have come up with smaller and smaller values as the accuracy has increased!

Most constants such as G have their size measured to obtain a value. It is not known why the value is what it is. Until Physicists understand why the constants in the Universe have the values that they do have, I don't think that we can assume them to be constant for all time.

I may be wrong though......

3. Nov 4, 2004

### pervect

Staff Emeritus
Well, let's take an example. Let's take the meter, as it used to be defined, a pair of marks on a bar.

We know that the universe is expanding relative to the meter. But how do we know that the meter isn't shrinking, and the universe is staying the same size?

Answer: we don't, not really. But if we are going to make measurements, we have to base them on something. So we define the meter as a constant.

Nowadays we don't use parallel marks on a bar, we've found it more convenient to define the meter as the distance that light travels in a certain amount of time. And we define time based on another property of matter, a certain number of cycles of the cesium atom undergoing a certain atomic transition.

So a lot of physical constants are just the necessary assumptions needed to measure things. Other times we get constants that are the result of experiment. In these cases we can say that the physical constants appear to be constant because of experiment - we measure them, very accurately, and we keep getting the same answer.

4. Nov 4, 2004

### rbj

i just want to build on what pervect is saying. you might want to take a look at the Planck Units or Natural Units page on Wikipedia: http://en.wikipedia.org/wiki/Planck_units . take a look at Planck Units and the Invariant Scaling of Nature.

anyway, the only quantities we measure (ultimately) in an experiment (or in our perception of reality) are *dimensionless* constants. if one of them changed beyond experimental error, we would know it. but if a dimensionful constant changes, such as G or c or h_bar or epsilon_0, we would not know the difference without measuring it against some other like dimensioned standard (and i can't think of a like dimensioned standard to measure G against that is more universal).

now certainly, the masses and sizes of particles and atoms, etc, can be expressed in terms of the Planck Mass or Planck Length and that would be a dimensionless constant. if that changed, we would know the difference. another important dimesionless constant is the Fine-Structure Constant, which is, from the POV of Natural Units is simply the square of the amount of charge of the electron (measured in units of the Planck Charge). since the electromagnetic force between two particles is proportional to the product of their charges, the Fine-Structure Constant (called "alpha") essentially defines the strength of the electromagnetic force relative to other forces.

r b-j

Last edited: Nov 4, 2004
5. Nov 6, 2004

### welcher_weg

thanks for replying guys.... so from what i gather, while can never be fully sure of whether constants are really constants on all possible scales, they are so intricately realted that if one changes our universe would change drastically.... hmmm...... lemme sleep over it!

6. Nov 6, 2004

### rbj

no. if a dimensionFUL constant (such as c, G, h_bar, etc.) changes but all dimensionless constants (alpha, any other ratio of like dimensioned quantities) remain constant, the universe does not change in any way that we could measure. those dimensionful values only take on the values that they do because of the anthropocentric definitions of units we use to measure them. if we measured everything in terms of Planck Units, then c=1, G=1, and h_bar=1. if God decides to change c, h_bar, and/or G, when we measure them in terms of Planck Units, they still come out to one. if ALL dimensionless constants remain the same value, than all lengths and distances will be the same multiple of the Planck Length that they used to be (for constant quantities anyway), all times, relative to the Planck Time, remain the same, and all masses, relative to the Planck Mass, are the same as before. if not, then a dimensionLESS quantity has changed and then we can measure it and see it is not the same. what c and G and h_bar (in human units such as SI or cgs) tell us is what the scaling of nature is - where Nature has put tick marks on her meter stick, her clock, and her weighing scale.